Estonian Journal of Earth Sciences, 2020, 69, 1, 2020–36–36 https://doi.org/10.3176/earth.2020.02

Muscle scars, mode of life and systematics of Pollicina (Mollusca) from the Ordovician of Baltica

John S. Peel

Department of Earth Sciences (Palaeobiology), Uppsala University, Villavägen 16, SE­75236 Uppsala, Sweden; [email protected]

Received 12 August 2019, accepted 17 October 2019, available online 19 December 2019

Abstract. Pollicina is a distinctive, but uncommon, univalved mollusc originally described from the Middle Ordovician (Darriwilian Stage) of the Baltic. The slender, bilaterally symmetrical shell expands slowly and is curved through up to about 90 degrees, but straightens in the latest growth stages. Pollicina corniculum, the type species from the St Petersburg region of Russia is redescribed, as is Pollicina crassitesta, the most common representative from the Tallinn area of Estonia. Muscle scars in P. crassitesta form a continuous circum­apertural band on internal moulds about half way between the apex and the apertural margin. The bilaterally symmetrical shell, orthocline aperture, circum­apertural muscle scar and frequent displacement of the apertural margin, as evidenced by dislocations in growth ornamentation, indicate that Pollicina lived as a limpet, clamped against the substrate. Suggestions that it was an open coiled gastropod lying on the sediment surface are rejected. As with most Ordovician limpet­formed shells, assignment of Pollicina to Tergomya or Gastropoda is equivocal, even controversial, not least on account of the tall shell and muscle band. Despite similarities with the tergomyan Cyrtolites, Pollicina is placed tentatively together with the archinacelloidean gastropods.

Key words: Mollusca, Gastropoda, muscle scars, habit, Ordovician, Darriwilian, Baltica.

INTRODUCTION been suggested that the earliest gastropods were limpet­ formed (Golikov & Starobogatov 1975; Haszprunar 1988; Univalved molluscs with cap­shaped or limpet­formed shells Parkhaev 2017). However, Lindberg (1988, 2008, 2009) have existed throughout the Phanerozoic. Vermeij (2017) and Ponder & Lindberg (1997) proposed that the ancestor calculated that this shell morphology has arisen of patellogastropod limpets was likely a coiled gastropod. independently more than 50 times in the evolutionary history The oldest confirmed patellogastropod is Triassic in age, of gastropods, and to this may be added a morass of based on studies of shell structure (Hedegaard et al. 1997), monoplacophoran and poorly known bilaterally symmetrical although numerous claims of Palaeozoic patellids exist in shells in the early–middle Palaeozoic (Runnegar & Jell 1976; historical and more recent literature, as discussed below. Peel 1991a, 1991b; Geyer 1994; Parkhaev 2008, 2017; Thus, the early history of undoubted patellogastropods, or Bouchet et al. 2017). Most familiar at the present day are the their supposed coiled ancestors, has not been established patellids which dominate rocky shores at intermediate in the Palaeozoic, almost half of their supposed geological latitudes and which can be used to illuminate the dilemma range, frustrated by the inadequacy of shell­based facing malacologists seeking to unravel gastropod information of the almost featureless shells and confusion evolutionary history. Despite a host of modern phylogenetic with morphologically similar shells of co­existing and developmental studies (Koufopanou et al. 1999; monoplacophoran mollusc groups (Peel 1991a, 1991b; Wanninger et al. 1999, 2000; Lindberg 2009; Kristof et al. Yochelson & Webers 2006). Fortunately, ongoing studies 2016), elucidation of their palaeontological record is of protoconchs and new phylogenetic approaches (Frýda frustrated by the morphological simplicity of their bilaterally et al. 2009; Frýda 2012) offer a potential way forward. symmetrical shell and muscle scar, the only characters that This paper examines a distinctive but poorly known are preserved in most fossil scenarios. Ordovician univalve which lies in limbo between Lindberg (1986, 1988) proposed that Patellogastropoda gastropods and monoplacophoran molluscs. Pollicina has was the sister group of other gastropods, although this proven to be enigmatic ever since the name was introduced hypothesis has since evolved (Zapata et al. 2014). It has by Holzapfel (1895) in a publication which described

© 2019 Author. This is an Open Access article distributed under the terms and conditions of the Creative Commons Attribution 4.0 International Licence (http://creativecommons.org/licenses/by/4.0).

20 J. S. Peel: Ordovician mollusc Pollicina

Devonian fossils from Germany. However, the designated described from the East Baltic. This Ordovician type type species of Pollicina is an Ordovician form originally species, Pollicina corniculum (Eichwald, 1860; Fig. 1), has

Fig. 1. Pollicina corniculum (Eichwald, 1860). Ordovician, Darriwilian, Kunda Stage, Popowka or Pulkowa [=Pulkovo], St Petersburg, Russia. A–F, lectotype, PSM 4/22: A, lateral view; B, supra­apical surface; C, lateral view; D, supra­apical surface at apex, with adherent matrix on right side; E, sub­apical surface; F, apertural view. G, H, replica of lectotype, PSM 4/22, showing prominent comarginal rugae: G, oblique lateral view; H, supra­apical surface. I, K, replica of paralectotype, PSM 4/23. J, CNIGRM 15692 (380/10903), sub­apical surface showing irregular longitudinal ridges. Arrow a locates equivalent points in B and H. Arrow b indicates equivalent points in C and G. Arrow c indicates the prominent transverse growth dislocation in A–C, H. Arrow r indicates ridges perpendicular to margin in I, J. Scale bar = 5 mm.

21 Estonian Journal of Earth Sciences, 2020, 69, 1, 20–36 been evaluated in several studies, (Knight 1941, 1952; ornamentation are lacking (Figs 2D–J, 3C–O), although Kisselev 1994; Evans & Cope 2003; Yochelson & Webers a rare exception is provided by the lectotype of Pollicina 2006) but the Devonian species proposed by Holzapfel crassitesta Koken, 1897 (Fig. 2A–C). This general lack (1895) has not. of information concerning ornamentation urges caution in The slowly expanding, slightly curved, bilaterally comparisons of internal moulds both with Pollicina symmetrical shell of Pollicina (Fig. 1) is universally corniculum and with internal moulds from other localities accepted as molluscan in origin but the position of the and horizons. The internal moulds are valuable, however, genus within Mollusca is controversial. Several of the in showing details of shell musculature that are not known various earlier interpretations as cephalopod, ‘hyolithid’ in Pollicina corniculum. pteropod, capulid gastropod, euomphaloidean gastropod, Pollicina is re­described on the basis of its two most monoplacophoran or the problematic xenoconchs and common species: Pollicina corniculum from the St toxeumorphorids were reviewed by Evans & Cope (2003) Petersburg region of Russia and Pollicina crassitesta who placed Pollicina within the monoplacophoran Class from Estonia. The latter material preserves muscle Tergomya Horný, 1965a sensu Peel 1991a. However, scars which promote a discussion of the systematic Yochelson & Webers (2006) maintained that Pollicina position of Pollicina. The distribution of shell was a gastropod. Bouchet et al. (2017, p. 194) noted that attachment muscles, together with the nature of growth usually Pollicina has been treated as a gastropod but that ornamentation in Pollicina corniculum, indicate that at this view had been rejected by Evans & Cope (2003). least some specimens of Pollicina lived as limpets The late Ellis L. Yochelson (1929–2006) nurtured an clamped against hard substrates. interest in Pollicina that extended over four decades and Repositories of figured specimens are indicated by the resulted in several unpublished manuscripts, aspects of following prefixes: CNIGRM, F. N. Chernyshev Central which were cited by Peel & Yochelson (1984), Evans & Geological Survey Research Museum, St Petersburg; Cope (2003) and Yochelson & Webers (2006). Yochelson ELM, Estonian Museum of Natural History, Tallinn; & Webers (2006, p. 25) summarized Yochelson’s theory PSM, Museum of the Department of Historical Geology, that Pollicina was an uncoiled or open coiled mollusc St Petersburg State University. related to euomphaloidean gastropods, an opinion that is not supported herein. However, Yochelson’s interest has materially advanced the present study since he passed on AUTHORSHIP OF POLLICINA to me photographs and replicas of the type material of Pollicina corniculum (Eichwald, 1860), the lectotype and Holzapfel (1895, pp. 182–183) attributed authorship of paralectotype of which are illustrated herein (Fig. 1). Pollicina to ‘Koken (in Litt.)’ quoting a written Yochelson’s photographs of the original specimens and communication from Ernst Koken in which the type was the replicas of these specimens were produced in designated as Cyrtolites laevis Eichwald from the Washington D.C. during the late 1980s and their identity Ordovician of Baltica. In this letter, Koken noted that he is confirmed by comparison with subsequent illustrations had used the name Pollicina in a manuscript, and it is of the types by Kisselev (1994). It is evident, however, reasonable to assume that this was the manuscript of that Yochelson’s photographs and the replicas were Koken (1897). Eichwald (1842, p. 71) had assigned prepared prior to the preparation of Kisselev’s (1994, Cyrtolites laeve to Cyrtoceras laeve [Sowerby in] fig. 1A–D) illustrations of the designated lectotype since Murchison, 1839 which he considered to be a cephalopod. that illustration figures a specimen which has been However, Eichwald (1860, p. 1048) recognized that his sectioned apically, along the plane of symmetry, species was distinct from Cyrtoceras laeve Sowerby in presumably to investigate the possible presence of septa. Murchison, 1839 (which he referred to as Cyrtolithes The replica of the lectotype also demonstrates the laevis) and delimited a new species as Cyrtolithes presence of prominent transverse rugae (Fig. 1G, H) not corniculum. His description lacked illustrations, referring clearly seen in the illustrations of Kisselev (1994) or the to Eichwald (1842, pl. 3, figs 5, 6), and he reinterpreted Yochelson photographs (Fig. 1A–C herein). his species as a pteropod related to hyolithids (Eichwald Comparison between Pollicina corniculum and most 1860). Cyrtolithes is an emendation of Cyrtolites Conrad, other material assigned to Pollicina is hindered by 1838 common in literature from the late 19th century. differences in preservation. Available specimens of Koken (1897, p. 197) clearly claimed authorship of Pollicina corniculum usually preserve the shell and Pollicina, referring to Pollicina Koken (in Holzapfel display the external ornamentation of sharp comarginal 1895) in his description of the genus. This description was lamellae (Fig. 1). Most of the other available specimens reproduced unchanged in the 1925 monograph Die assigned to the genus by Koken (1897) and Koken & Gastropoden des baltischen Untersilurs. However, the Perner (1925) are internal moulds where details of authorship of this work is tainted by uncertainty reflecting

22 J. S. Peel: Ordovician mollusc Pollicina

Fig. 2. Pollicina crassitesta Koken, 1897. Ordovician, Darriwilian, Kunda Stage, Tallinn, Estonia. A–C, ELM g8:23 [1208g8/23], lectotype, internal mould with adherent ornamented shell patches and diagenetic outer layer (dol), Domberg (=Toompea): A, supra­ apical surface; B, lateral view; C, detail of lateral surface to show ornamentation. D–G, ELM g8:7 [1208g8/7], internal mould lacking apex, Springthal (=Tondi): D, apical view showing elliptical scar after the broken apex; E, supra­apical surface with median spiral trace arrowed; F, sub­apical surface; G, lateral view. H–J, ELM g8:22 [1208g8/22], internal mould with broken apex, Springthal (=Tondi): H, supra­apical surface; I, sub­apical surface with elliptical scar after broken apex; J, lateral view. Scale bars = 5 mm. the degree of editing and collation of several texts carried 1999; Isakar & Ebbestad 2000; Evans & Cope 2003; Peel out by Jaroslav Perner which had been prepared (and in 2019). The last option, as Koken & Perner (1925), is some cases printed) years before by Ernst Koken (1860– followed here, although acts which are clearly referable 1912). Perner also authored supplementary material to a specific author are noted. prepared after Koken’s death. New taxa in the monograph Knight (1941, p. 264, footnote) considered the assignment are attributed individually both to Koken and Perner but of authorship of Pollicina to Koken by Holzapfel (1895) to delimitation of their individual contributions is often be just a matter of courtesy and he regarded therefore uncertain. The work has been cited as by Koken alone Holzapfel (1895) as the author of Pollicina. Knight (1941, (Yochelson 1963; Wahlman 1992; Bouchet et al. 2017), pl. 5, fig. 2; see also Knight & Yochelson 1960) reproduced as Koken edited by Perner (Knight 1941; Knight et al. Eichwald’s (1842) illustration since he was unable to locate 1960) or Koken & Perner together (Wenz 1938; Ebbestad the type material of Pollicina. Knight’s (1941) practice

23 Estonian Journal of Earth Sciences, 2020, 69, 1, 20–36

Fig. 3. Pollicina Koken in Holzapfel, 1895. Ordovician, Darriwilian, Kunda Stage. A, B, Pollicina corniculum (Eichwald, 1860), Pulkowa (=Pulkovo), St Petersburg, Russia: A, CNIGRM 15702 (200/10903), lateral view with deep scratches resulting from preparation on right side; B, CNIGRM 15703 (201/10903), lateral view showing dislocation of growth lines, arrow. C–N, Pollicina crassitesta Koken, 1897, internal moulds. C–D, CNIGRM 15600 (394/10903), lateral view with small scars arrowed, Laaksberg (=Lasnamägi), Tallinn. E–G, CNIGRM 15597 (392/10903), Laaksberg (=Lasnamägi), Tallinn: E, sub­apical surface with median ridge–furrow complex arrowed; F, lateral view; G, supra­apical surface. H–J, N, ELM g1:2323, Tallinn: H, oblique view of supra­apical surface (see detail in N); I, lateral view; J, oblique view of sub­apical surface; N, detail of muscle band on supra­apical surface with median plane of symmetry indicated. L, M, ELM G1:2919, Tallinn: L, lateral view; M, sub­apical view. K, O, Pollicina sp., ELM G1:353, internal mould, Estonia, precise locality unknown: K, lateral view; O, detail of muscle band. Scale bars = 5 mm (E–G), otherwise 4 mm.

24 J. S. Peel: Ordovician mollusc Pollicina concerning authorship has been followed by most later (1941), Knight & Yochelson (1960), Kisselev (1994) and authors (Knight & Yochelson 1960; Kisselev 1994; Evans & Evans & Cope (2003).

Cope 2003; Yochelson & Webers 2006). However, Bouchet Other species. Koken (1897) and Koken & Perner (1925) et al. (2005, 2017) followed Wenz (1938, p. 87) in considering described several Ordovician species of Pollicina, most ‘Koken (in Holzapfel), 1895’ to be the author. This action is of which are based on internal moulds in limestone from followed here, rephrased as Pollicina Koken in Holzapfel, Estonia, northwest Russia and Sweden, and from glacial 1895. erratics in northern Germany. A revision of the status of Perner (in Koken & Perner 1925) repeatedly all these is not attempted herein, but Pollicina ampliata employed the spelling Policina. Yochelson & Webers Koken in Koken & Perner, 1925 from the Sandbian at (2006, p. 25) inadvertantly referred to Pollicinia. Raasiku (= Rasik), Estonia, is much younger and more In the main text of Koken & Perner (1925, p. 325), rapidly expanding than other species and should be Pollicina is referred to the gastropod ‘Family Capulidae’, excluded from the genus. Pollicina conoidea Koken in but a footnote authored by Perner (Koken & Perner 1925, Koken & Perner, 1925 was transferred to Hypseloconus? p. 227) proposed a new capuliform family, the Family conoidea by Yochelson (1963) and to Pygmaeoconus Pollicinidae. In his study of scaphopod­like molluscs, conoideus by Yochelson (1977). Two other new species, Starobogatov (1974) independently proposed Pollicinidae Pollicina acuta and Pollicina brevis, lack illustrations and as a new family within the Order Toxeumorphorida Perner in Koken & Perner (1925) commented that he Shimansky, 1962 of the molluscan Class Xenoconchia could not locate the original specimens. Shimansky, 1963, although he regarded the xenoconchs Evans & Cope (2003) assigned flattened specimens as a subclass within the Class Solenoconchia Lacaze­ from the middle Ordovician of the United Kingdom to Duthiers, 1857, a little used appellation largely equivalent Pollicina corniculum. Tentative identifications of to Scaphopoda Bronn, 1862 (Peel & Yochelson 1984; Pollicina from the lower Cambrian of Denmark and Steiner & Kabat 2001). Kisselev (1994) recognized southern Sweden by Poulsen (1967) and from the middle Perner as the author of Pollicinidae but he retained the Cambrian of Australia by Runnegar & Jell (1976) are assignment to Xenoconchia, Toxeumorphorida. Peel & discounted. Yochelson (1984) abandoned the Class Xenoconchia and This redescription of Pollicina is based on type and relocated Toxeumorphorida as a third order within the additional material of Pollicina corniculum from Russia Class Hyolitha Marek, 1963, alongside Hyolithida and Pollicina crassitesta Koken, 1897 from Estonia. Syssoiev, 1957 and Orthothecida Marek, 1966, but excluded Pollicina. Evans & Cope (2003, p. 145) Diagnosis. Bilaterally symmetrical, slowly expanding regarded Starobogatov (1974) as the author of cyrtoconic shell which is slightly curved or coiled through Pollicinidae which they regarded as tergomyan up to about 90 degrees; apex blunt, overhanging the sub­ monoplacophoran. Bouchet et al. (2017, p. 194) affirmed apical surface. Aperture orthocline or nearly so, oval to ‘Perner, 1925 (in Koken)’ as the author of the family, circular in cross section, with its maximum dimension less rephrased herein as Pollicinidae Perner in Koken & than half the total height of the shell. Ornamentation of Perner, 1925. comarginal growth lines that vary from slightly lamellose to fine and even, but may be rugose and with frequent transverse irregularities. Muscle scar is a circum­apertural SYSTEMATIC PALAEONTOLOGY band located at about half the distance from the apertural margin to the apex.

Family POLLICINIDAE Perner in Koken & Perner, Discussion. Specimens of Pollicina corniculum from 1925 Russia and crushed material figured by Evans & Cope Genus Pollicina Koken in Holzapfel, 1895 (2003) have a bluntly rounded apex but details of the Type species. Cyrtolithes corniculum Eichwald, 1860 (= protoconch are not well known, as is the case with most Cyrtoceras laeve Eichwald, 1842 non Sowerby in Ordovician limpet­formed shells. Internal moulds of Murchison, 1839) from the Ordovician (Darriwilian Pollicina may be rounded at the apex as a result of apical Stage) of the East Baltic. Eichwald’s (1860) species shell thickening or development of an apical plug. Cyrtolithes corniculum was formally fixed as type species Septation of the most apical part was clearly illustrated of Pollicina by Bouchet et al. (2017) under the name by Evans & Cope (2003, text­fig. 4). Cyrtolites corniculum Eichwald, 1860, with reference to In terms of its cyrtoconic shell form, Pollicina is Article 70.3 of the International Code of Zoological similar to several Cambrian (Miaolingian and Furongian) Nomenclature (ICZN 2012), in accordance with earlier molluscs (Webers et al. 1992; Stinchcomb & Angeli 2002; usage by Koken (1897), Koken & Perner (1925), Knight Yochelson & Webers 2006) often interpreted as

25 Estonian Journal of Earth Sciences, 2020, 69, 1, 20–36 hypseloconidan tergomyans (Peel 1991a, 1991b), but the from the Eminence Formation (Cambrian, Furongian relationships of many of these univalved shells from the Series) of Missouri, has an elliptical aperture and late Cambrian–Ordovician and their classification are prominent longitudinal ribs not seen in Pollicina. Similar equivocal (Yochelson & Webers 2006; Bouchet et al. specimens were described from the Cambrian (Furongian 2017). Characteristically, the apex in tall, slowly expanding Series) of Minnesota by Yochelson & Webers (2006). hypseloconidan shells does not overhang the sub­apical In terms of gross form of the tall, slowly expanding margin (Yochelson & Webers 2006, p. 22, fig. 8), as it shell, Pollicina superficially resembles some specimens does in Pollicina. assigned to the Silurian–Devonian platyceratoidean Knightoconus Yochelson, Flower & Webers, 1973, in Orthonychia Hall, 1843, not least the type species lateral perspective, has convex and concave surfaces Orthonychia subrecta Hall, 1843 as illustrated by Knight which are superficially similar to the supra­apical and (1941, pl. 88, fig. 5). However, the great majority of the sub­apical surfaces of Pollicina but illustrations by numerous Devonian specimens assigned to Orthonychia Webers & Yochelson (1989) and Webers et al. (1992) lack bilateral symmetry and are often irregular and twisted indicate that coiling in Knightoconus is sigmoidal during in their later growth (Perner 1903; Frýda et al. 2008, ontogeny. Thus, the initially convex supra­apical surface 2009). of the juvenile Knightoconus becomes concave in later Pollicina corniculum (Eichwald, 1860) growth stages and is therefore not equivalent in terms of Figures 1A–K, 3A, B coiling direction to the sub­apical surface of Pollicina. The convex surface of the adult Knightoconus is derived 1842 Cyrtoceras laeve Eichwald, p. 71, pl. 3, figs 5, 6 from the sub­apical surface of the juvenile and not equivalent (non Sowerby in Murchison 1839). to the supra­apical surface of Pollicina. Juvenile shells are 1860 Cyrtolithes corniculum Eichwald, p. 1048. not known from most hypseloconids but their form can 1897 Pollicina corniculum, Koken, p. 197. be interpreted as in Knightoconus. In many, the aperture 1925 Pollicina corniculum, Koken & Perner, p. 228, narrows beneath the convex surface while it is sub­ pl. 38, figs 8, 12, 13. circular in Pollicina. 1994 Pollicina corniculum, Kisselev, p. 36, fig. 1A–F. Some specimens originally assigned to Shelbyoceras 2003 Pollicina corniculum, Evans & Cope, p. 145, Ulrich & Foerste in Bridge, 1930, especially Shelbyoceras text­fig. 4. bessemerense Ulrich, Foerste & Miller, 1943 from the Type material. PSM 4/22 (lectotype; Fig. 1A–H) and Cambrian (Furongian Series) of Alabama, which was PSM 4/23 (paralectotype; Fig. 1I, K) designated by relocated to Hypseloconus bessemerense by Stinchcomb & Echols (1966) and later partly to Gayneconus Kisselev (1994). Ordovician, Darriwilian, Kunda Stage. Stinchcomb & Angeli, 2002, also have a cyrtoconic shell. Eichwald (1842) gave the locality as Popowka [Popovka Shelbyoceras was first described as a cephalopod but River] but subsequent authors and specimen labels refer reinterpreted as a septate monoplacophoran mollusc by to the locality as Pulkowa (Koken 1897; Koken & Perner Stinchcomb (1980). However, most Shelbyoceras are 1925) or Pulkova, St Petersburg, Russia. Kisselev (1994) more rapidly expanding with prominent comarginal and Evans & Cope (2003) refer to Polkovo, the current costae on lateral surfaces. The plane of the aperture is Russian spelling. orthocline in Pollicina such that the apex overhangs the Additional figured material. CNIGRM 15702 (200/10903; subapical margin. It is oblique in specimens of Gayneconus Fig. 3A) and CNIGRM 15703 (201/10903; Fig. 3B), bessemerense (Stinchcomb & Echols 1966; Stinchcomb & specimens illustrated by Koken & Perner (1925). Angeli 2002) and in the type species of Shelbyoceras CNIGRM 15692 (380/10903; Fig. 1J). Ordovician, illustrated by Stinchcomb (1980), such that the apex does Darriwilian, Kunda Stage, Pulkowa [Pulkovo], St not overhang the subapical margin. In Shelbyoceras Petersburg. bigpineyensis Stinchcomb, 1986, from the upper Other material. More than 10 specimens from Pulkovo, Gasconade Formation of Missouri, the aperture is elliptical, labelled Pulkowa, in the collection of the F. N. width about two thirds of length, compared to slightly Chernyshev Central Geological Survey Research oval or sub­circular in Pollicina. Unlike other species of Museum (CNIGRM), St Petersburg, Russia. Shelbyoceras, septation is restricted to the tip, as in Pollicina. Diagnosis. Type species of Pollicina in which the The apertural margin is almost orthocline in slowly expanding cyrtoconic shell is often uniformly Ulrichoconus Stinchcomb & Angeli, 2002, a robust, curved through 90 degrees (Figs 1A, 3A). Shell thin, septate shell from the Cambrian (late Miaolingian Series) but thickening near the aperture; ornamentation of of Missouri, USA (Stinchcomb & Angeli 2002, figs 2.1– thin comarginal lamellae that are slightly flared and 2.3). Orthoconus Stinchcomb, 1986, originally described irregular.

26 J. S. Peel: Ordovician mollusc Pollicina

Description. The slowly expanding cyrtoconic shell is surface, with the apex tilted towards the viewer when curved usually through about 90 degrees such that the compared to Fig. 1C; this orientation causes the shell to apex overhangs the sub­apical margin (Fig. 1C). The appear less strongly coiled and more rapidly expanding greatest dimension of the slightly oval aperture in the than in a true lateral view (Fig. 1C). Corresponding points lectotype (Fig. 1F), lying within the plane of symmetry of are indicated by arrows (Fig. 1C, G, arrow b). Equivalent the shell, is almost half the total height (Fig. 1A). The rugae are visible in the paralectotype (Kisselev 1994, shell coil straightens and reduces its rate of expansion in fig. 1E; Fig. 1I, K). larger specimens, as the aperture is approached, such that Prominent comarginal discontinuities in growth are the maximum apertural dimension is only about one third present in the lectotype (Fig. 1A–C, H, arrow c) and of the total height (Fig. 3B). In cross section, the shell is specimens in the CNIGRM collection in St Petersburg oval even at the earliest preserved stages (Fig. 1D, F, H). (Fig. 3B, arrow). Several specimens also show short The apertural margin is orthocline and may be slightly ridges perpendicular to the growth ornamentation flared to form irregular rugae in the latest growth stages (Fig. 1I, J, arrow r). Minor irregularities in the spacing (Fig. 1G–I). Ornamentation consists of closely spaced, of the growth lines are numerous but evidence of acute, comarginal growth lines which are usually slightly breakage and repair of the apertural margins has not lamellose and irregular (Fig. 3A, B), and may be been observed. interrupted by discontinuities (Fig. 1A–C, H, arrow c). In Evans & Cope (2003) attributed several specimens most specimens the shell is thin but the lectotype shows from Darriwilian strata in the United Kingdom to thickening near the aperture (Fig. 1A). Pollicina corniculum. The specimens are crushed in mudrocks, which hinders their precise identification to Discussion. The fact that Eichwald (1842) gave the locality species, but internal closure of the apex by a transverse as Popowka, and not Pulkowa, as used by Koken (1897) septum is seen in some illustrated examples. Evans & and Koken & Perner (1925), or Pulkovo, as used by Cope (2003) considered the small conical portion of the Kisselev (1994) and Evans & Cope (2003), casts a measure shell lying adapical of this septum to be a protoconch of doubt on the designation by Kisselev (1994) of the but details of its morphology are not well known. Septation lectotype. Popowka lies a few kilometres to the southeast has not been demonstrated in available specimens of of Pulkovo, to the south of St Petersburg. However, that Pollicina corniculum from the East Baltic which usually specimen (illustrated here as Fig. 1G) agrees closely with preserve the outer shell, although the apex is often not the engraving given by Eichwald (1842, pl. 3, fig. 5), so the well preserved. The apex is broken away in most identification is maintained despite the confusion. available internal moulds in limestone of Pollicina but, Illustrations of the lectotype and paralectotype of unsurprisingly in such a narrow shell, septation, the Pollicina corniculum given by Kisselev (1994, fig. 1) are presence of apical thickening or an apical plug has been accompanied by incorrect scale information in the figure widely observed. Koken & Perner (1925, p. 228) caption. This error was inadvertently perpetuated by Evans recorded but did not illustrate septation in Pollicina acuta & Cope (2003, p. 143). Kisselev (1994) stated that the Koken, 1897 from the Darriwilian of Dalarna, Sweden. magnification of the lateral view of the lectotype and paralectotype is × 5, indicating a specimen height of only Pollicina crassitesta Koken, 1897 Figures 2A–J, 3C–J, L–N 9 mm for the lectotype. Measurements of the replicas and scales on photographs supplied by E. L. Yochelson indicate 1897 Pollicina crassitesta Koken, p. 198. that these specimens are more than twice this size (Fig. 1). 1925 Pollicina crassitesta; Koken & Perner, p. 229, pl. Perception of the form of the shell in illustrations of 38, figs 1–3, 9–10, 11?. the lectotype of Pollicina corniculum (Fig. 1A–H) is susceptible to differences in orientation and lighting. Type material. ELM g8:23 [1208g8/23], here designated Thus, the prominent transverse rugae visible near the lectotype, original of Koken (1897, p. 198), the same aperture in recently prepared illustrations of the replica specimen illustrated by Koken & Perner (1925, pl. 38, (Fig. 1G, H) are only poorly visible in corresponding fig. 9; Fig. 2A–C), Domberg (=Toompea), Tallinn. Ordovician, Darriwilian, Kunda Stage, Estonia. illustrations of the lectotype itself prepared by Yochelson (Fig. 1B, C) and Kisselev (1994). Lighting of the supra­ Other figured material. ELM g8:22 [1208g8/22], original apical surface in Fig. 1H is strongly oblique to emphasize of Koken & Perner (1925, pl. 38, fig. 10; Fig. 2H–J), the rugae. These appear less marked when the specimen Springthal (=Tondi), Tallinn. ELM g8:7 [1208g8/7], has been rotated slightly and lighting is uniform (Fig. 1B). Springthal (=Tondi), Tallinn, original of Koken & Perner Corresponding points are indicated by arrows (Fig. 1B, (1925, pl. 38, figs 1, 2; Fig. 2D–G). The specimen H, arrow a). Figure 1G is illuminated with oblique illustrated as Koken & Perner (1925, pl. 38, fig. 11) has lighting, and the specimen is oriented oblique to the lateral not been identified.

27 Estonian Journal of Earth Sciences, 2020, 69, 1, 20–36

CNIGRM 15597 (392/10903), Laaksberg (=Lasna­ Koken (1897) and Koken & Perner (1925) described mägi), Tallinn. CNIGRM 15600 (394/10903), Laaksberg a thin outer shell layer overlying a strongly ornamented (=Lasnamägi), Tallinn. ELM g1:2323, Tallinn. ELM and thicker inner layer as characteristic of Pollicina g1:2919, Tallinn. Ordovician, Darriwilian, Kunda Stage, crassitesta and this layer is emphasized in the original Estonia. sketch (Koken 1897, p. 198). The thin outer layer is

clearly visible in the lectotype (Fig. 2A–C, dol) where it Diagnosis. Species of Pollicina in which the laterally covers prominent growth ornamentation and is evidently compressed apex of the internal mould is elongate elliptical diagenetic in origin. in cross section with its long axis on the plane of symmetry. Pollicina crassitesta is delimited from most specimens Shell thick; ornamentation of prominent comarginal lamellae of Pollicina corniculum in the F. N. Chernyshev Central and fine growth lines crossed by spiral cords separated by fine Geological Survey Research Museum, St Petersburg, by grooves. its uniformly thick shell, although the shell of the Description. The slowly expanding, bilaterally symmetrical, lectotype of Pollicina corniculum is also relatively thick cyrtoconic shell is curved usually through about 60 degrees, in its latest growth stages (Fig. 1A). The lateral surfaces producing a tall shell in which the apex overhangs the sub­ are flattened in the earliest preserved growth stages in apical margin, although the earliest growth stage is not Pollicina crassitesta such that the length of the whorl known. In lateral view (Figs 2G, 3C) the maximum cross section is about twice the width (Fig. 2D, I) whereas dimension of the aperture, measured in the plane of the cross section is almost circular in Pollicina symmetry, is usually less than half the total height. The cross corniculum. Spiral cords cross the prominent growth section of the internal mould is strongly elliptical at the ornamentation in the lectotype of Pollicina crassitesta earliest preserved growth stage, its maximum dimension (Fig. 2A–C) but are absent in Pollicina corniculum. lying within the plane of symmetry of the shell and being twice the width (Fig. 2D, F, I); it becomes proportionately POLLICINA wider, oval, in later growth stages. The apertural margin is MUSCLE SCARS IN orthocline and periodically the outer shell surface may be slightly constricted near the aperture (Fig. 2A–C). Near the Internal moulds of Pollicina corniculum with muscle aperture, ornamentation of the thick shell consists of scars have not been identified. The following description is based on internal moulds of Pollicina crassitesta relatively broad, flat­topped, comarginal growth lamellae collected in Estonia. When preserved, the muscle that are separated by narrower U­shaped grooves, both attachment area forms a raised, rather broad, comarginal marked with fine growth lines (Fig. 2C); they are crossed band that is most conspicuous on the lateral surfaces but by numerous fine spiral cords. Internal moulds are generally usually can be traced around the sub­apical and supra­ smooth, without indications of transverse rugae, although a apical surfaces (Fig. 3C–J, L–N). In lateral view, the band circum­apertural raised band of muscle scars may be usually occurs at just below half the height of the internal prominent at about half the distance from the apex to the mould, but variation in the placement of the band in part aperture (Fig. 3C, E–I). reflects breakage of the aperture and probably differences Discussion. Pollicina crassitesta was illustrated by Koken in the rate of expansion of the shell cone (compare (1897, p. 198) with a schematic drawing based on the Fig. 3C, K, L). The band is most weakly expressed near specimen here designated as lectotype (Koken & Perner the median plane of the supra­apical surface where it may 1925, pl. 38, fig. 9; Fig. 2A–C). Siegfried (1935, p. 145) intersect one or more obscure spiral ridges, the ridge– listed the figured specimens of Koken & Perner (1925, furrow complex (Fig. 3E, arrow). In lateral view, the band pl. 38, figs 1–3, 9–11) as paratypes, commenting that one is inclined towards the sub­apical surface (Fig. 3F), of these specimens should be selected as lectotype. The corresponding to the inclination of the apertural margin location of the specimen in figure 11 was not known to in its current location, and not parallel with the apertural Siegfried (1935) and it was not available in the present plane of the latest growth stage. study. These specimens are not paratypes, since the author The muscle band in CNIGRM 15597 (392/10903) of Pollicina crassitesta was Koken (1897) and not Koken from Lasnamägi (Fig. 3E–G) is continuous but takes a & Perner (1925). Of the specimens mentioned by Siegfried slight step towards the aperture on the median portion of (1935), only that illustrated by Koken & Perner (1925, the supra­apical surface, adjacent to the ridge–furrow pl. 38, fig. 9; Fig. 2A–C) can be demonstrated conclusively complex (arrow in Fig. 3E). The same deflection is to have been part of Koken’s (1897) original lot. Koken apparent in CNIGRM 15600 (394/10903), also from (1897) did refer, however, to other specimens in his Lasnamägi, where the previously broad band appears to original lot, so the sole illustrated specimen cannot be give way to several small scars as the median plane is assumed to be the holotype by monotypy. approached (Fig. 3C, D). Similar displacement is also

28 J. S. Peel: Ordovician mollusc Pollicina evident in ELM g1:2323 from Tallinn (Fig. 3H–J, N) (CNIGRM), St Petersburg, do not show the asymmetry where a slight notch and raised section of the band are that Yochelson & Webers (2006) cited. visible in the adapical margin of the muscle on each side The interpreted limpet­formed mode of life of of the plane of symmetry (Fig. 3N). The muscle Pollicina is supported by the frequent displacement of the attachment scar is also narrower below the apex (Fig. 3J), apertural margin witnessed by disturbances in growth lines as in a second specimen from the same locality (Fig. 3M). and the periodic development of transverse rugae in The shell in ELM g1:353 (Fig. 3K) is more slowly specimens of Pollicina corniculum (Fig. 1A, H–K, 3B). expanding than other illustrated internal moulds, in which Similar displacements appear to be present in crushed respect it is similar to a figured specimen of Pollicina specimens assigned to Pollicina corniculum from the corniculum (Fig. 3B). In contrast to other illustrated Ordovician of the United Kingdom by Evans & Cope muscle scars, the band here is slightly depressed into the (2003) but specimens of other species described by Koken surface of the internal mould and its surface is marked (1897) and Koken & Perner (1925) are mainly internal with several comarginal grooves and ridges (Fig. 3O). The moulds lacking information concerning the outer surface precise collection locality of this specimen in Estonia is of the shell. Apart from infrequent minor chipping, the not known, but it is labelled Pollicina cyathina Koken, displacements in Pollicina corniculum from Pulkovo do 1897, a species that was originally described from not show penetrative scars and repaired injuries which Rättvik, in Dalarna, Sweden. It is here referred to as could be attributed to physical breakage or failed predatory Pollicina sp. attacks (Ebbestad & Peel 1997; Alexander & Dietl 2003; Lindström & Peel 2005; Ebbestad et al. 2009). They reflect re­positioning of the shell aperture against a hard substrate, MODE OF LIFE OF POLLICINA as is common in many platyceratid gastropods (Perner 1903; Bowsher 1955; Rollins & Brezinski 1988; Boucot The circum­apertural distribution of muscle scars in 1990; Horný 2000; Gahn & Baumiller 2003; Frýda et al. Pollicina crassistesta indicates that it clamped the shell 2008). Many of these platyceratids also show longitudinal aperture against the substrate when disturbed, invalidating ridges or folds as a result of their accomodation of the suggestion by Yochelson & Webers (2006) that it was irregularities in the surface to which they attach. Ridges an open coiled gastropod which would have lain on the of this kind are present also in some specimens of Pollicina sediment surface on its lateral area, with the axis of corniculum (arrows in Fig. 1I, J). Clearly, this is not to coiling and plane of the aperture vertical. Peel (2019) imply that the isostrophic Pollicina is related to described muscle scars in the Ordovician euomphaline anisostrophic platyceratid gastropods (Frýda et al. 2009), gastropods Asgardaspira Wagner, 2002 and Lesuerilla although its placement together with platyceratids by Koken, 1896 from Baltica which undoubtedly lived with Koken (1897) and Koken (in Koken & Perner 1925) is their base on or in the sediment surface and with a vertical possibly indicative of his views concerning its mode of axis of coiling in the manner envisaged for Pollicina by life. Not least, the muscle scar patterns of the two groups Yochelson & Webers (2006). In each of these genera, the are usually quite different (Horný 1964; Mazaev 1996; muscle attachment area is reduced and restricted to the Frýda et al. 2008), although U­shaped muscle scars have umbilical shoulder of the whorl (Peel 2019), quite unlike been described in tall, slowly expanding, specimens of the circum­apertural distribution of muscle scars seen in Orthonychia by Rollins & Brezinski (1988). Neither is Pollicina. Placement of the scars in Asgardaspira and there any supporting evidence that Pollicina corniculum Lesuerilla more than half a whorl back from the aperture enjoyed a symbiotic relationship with echinoderms, as is indicates deep withdrawal of the soft parts into the narrow the case with many platyceratids from the Ordovician to shells. The aperture in Asgardaspira and Lesuerilla was Permian (Bowsher 1955; Rollins & Brezinski 1988; most probably closed by an operculum but if developed Boucot 1990; Gahn & Baumiller 2003), although during ontogeny this would have been superfluous in platyceratids may be associated with hard substrates other adult Pollicina, as in present­day limpets (Vermeij 2017). than echinoderms (Horný 2000). Yochelson (in Peel & Yochelson 1984, p. 218) supported Stinchcomb (1975) and Stinchcomb & Angeli (2002) interpretation of Pollicina as an open coiled gastropod, noted the correlation of hypseloconids with digitate claiming that coiling in Pollicina corniculum deviated slightly stromatolites in shallow water carbonates of Cambrian– from bilateral symmetry. Yochelson (in Yochelson & Webers Ordovician boundary strata in Missouri, although the 2006, p. 25) emphasized this supposed anisostrophy with septate Shelbyoceras was not associated with stromatolites. reference to material from Estonia, exactly the same Taking into account the tallness of the shells relative to the specimens that are reviewed herein. However, these small size of the foot, it seems that the stromatolite buildups specimens and material from Estonia in the F. N. may have offered sheltered living sites, at least in Chernyshev Central Geological Survey Research Museum comparison to the high energy patellid­dominated rocky

29 Estonian Journal of Earth Sciences, 2020, 69, 1, 20–36 shore environments of the present day. However, Vermeij Evans & Cope (2003) did not make close comparison (2017) has pointed out that these familiar patellid with limpet­formed lower Palaeozoic supposed gastropods, communities of middle latitudes represent an adaptation to such as Archinacella Ulrich & Scofield 1897 or resist predation and that many limpets occur in less Floripatella Yochelson, 1988. They presented a detailed dynamic settings. Yochelson & Webers (2006) cast doubt comparison of Pollicina with hypseloconid tergomyans on the stability of the limpet interpretation of the slender (Peel 1991a, 1991b), but preferred assignment to the Pollicina shell in their striving to find evidence of cyrtonellid tergomyans on the basis of general shell form. anisostrophism. It is relevant to note here that Parkhaev (in Bouchet et al. It is clear that the slender shell form of Pollicina is 2017) distributed cyrtonellids sensu Peel (1991a, 1991b) not characteristic of limpets at the present day or through between three subclasses of untorted molluscs. The geological time, although numerous univalves in just the arguments presented by Evans & Cope (2003) are well late Cambrian–early Ordovician approach such a form founded, although it should be remembered that many (Stinchcomb & Echols 1966; Stinchcomb 1980; Webers of the molluscs described by Stinchcomb (1980), & Yochelson 1989; Webers et al. 1992; Stinchcomb & Stinchcomb & Echols (1966), Stinchcomb & Angeli Angeli 2002; Yochelson & Webers 2006). A loose (2002) and Yochelson & Webers (2006) are poorly parallel can be drawn with the widespread development known with regard to their ontogeny and muscle scars. of irregular, tall, clamping shells of Orthonychia amongst Cyrtolites, as described by Horný (1965a, 1965b) from the platyceratoidean gastropods in certain mainly the Middle Ordovician of Canada, is regularly coiled Devonian environments (Perner 1903; Frýda et al. 2008, through several whorls, unlike the cyrtoconic Pollicina, 2009). and its shell has a greater rate of whorl expansion. Discrete paired muscle scars are distributed in a broad belt across the supra­apical surface and continue as a ribbon­ SYSTEMATIC POSITION OF POLLICINA like band beneath the apex on the sub­apical surface (Horný 1965a; Fig. 4A). The pattern is diffuse when Opinions that Pollicina was a cephalopod (Eichwald 1842), hyolith or toxeumorphorid xenoconch (Eichwald 1860; Starobogatov 1974; Kisselev 1994) are readily rejected, as discussed in part by Evans & Cope (2003). In reviewing Yochelson’s suggestion (in Peel & Yochelson 1984) that Pollicina was an open coiled gastropod, Evans & Cope (2003) inferred a lack of the characteristic gastropod anisostrophic coiling in the crushed material that they assigned to Pollicina. Furthermore, they considered that the blunt apex indicated a protoconch much larger than that of gastropods, without any suggestion of torsion (Evans & Cope 2003). Yochelson & Webers (2006) did not respond to the comments of Evans & Cope (2003) but restated the open coiled gastropod hypothesis. Evidence concerning the nature of the protoconch of Pollicina is currently inconclusive. However, the present paper discounts the shell anisostrophism claimed by Yochelson & Webers (2006) and describes muscle scar patterns indicative of a limpet­ formed mode of life. Thus, their interpretation of Pollicina as an open coiled gastropod, lying on its side in the manner of euomphaloideans, is rejected. The issue of preserved muscle scars in hypseloconid tergomyans is controversial and most relevant material is poorly preserved. Yochelson & Webers (2006) reviewed various studies but were generally negative to claims of serially arranged scars, although such were described by Stinchcomb (1980). Dzik (2010) presented a provocative Fig. 4. Muscle scar patterns in Ordovician univalve molluscs. analysis of Ordovician shells with serially arranged Cyrtolites (A) and Pollicina (B) intepreted as Tergomya. muscle scars, but none resemble Pollicina. Pollicina (C) and Archinacella (D) interpreted as Gastropoda.

30 J. S. Peel: Ordovician mollusc Pollicina compared to the circum­apertural muscle band in Pollicina is not seen in other putative gastropod limpets Pollicina crassitesta (Fig. 3C, H, L). However, as noted from the Palaeozoic to the present day, although it was by Bandel (1982), Harper & Rollins (1982, 2000), Peel present in some late Cambrian–middle Ordovician (1991a, 1991b) and others, the distribution of muscle tergomyan groups (Stinchcomb & Echols 1966; scars in cap­shaped molluscs is largely a reflection of their Stinchcomb 1980; Webers & Yochelson 1989; Webers et function. Thus, the compact circum­apertural muscle scar al. 1992; Stinchcomb & Angeli 2002). of Pollicina may result from consolidation of more widely Archinacella and its near relatives are not the only dispersed muscle attachment sites such as those seen in Ordovician cap­shaped fossils that have been interpreted Cyrtolites, reflecting the specific mechanical requirements as gastropods (Ulrich & Scofield 1897; Koken & Perner of clamping the tall, narrow shell perpendicularly against 1925; Horný 1963, 1997; Yochelson 1988, 1994). In older its substrate. Points of similarity include the differ­ literature (e.g. Billings 1861–1865, see Bassler 1915), entiation of scars on the supra­apical surface, immediately many were referred to Metoptoma Phillips, 1836, which to either side of the median ridge–furrow complex in was originally described from the Carboniferous and Pollicina (Fig. 3C, E) and in cyrtonellids (Horný 1965b, interpreted as a gastropod by Knight et al. (1960), while figs 2, 5). Archinacella was interpreted in the same volume as a The muscle scar pattern in Sinuitopsis acutilira Hall, monoplacophoran mollusc (Knight & Yochelson 1960). 1861 from the Devonian of New York State, USA Starobogatov & Mazaev (1999) transferred Metoptoma (Rollins & Batten 1968, text­fig. 1) is more consolidated to the archinacelloids. Bouchet et al. (2017) regarded than the widespread pattern in Cyrtolites. Thus, it more metoptomatids and archinacelloideans as Palaeozoic closely resembles the pattern seen in Pollicina in that molluscs of uncertain position within Mollusca elongate lateral and latero­ventral scars form a more or (Gastropoda or Monoplacophora). less continuous U­shaped band passing beneath the Yochelson (1988) described Floripatella from the overhanging apex. A pair of dorsal scars and the Kanosh Shale (Ordovician, Dapingian–Darriwilian) of median ridge–furrow complex lie between the prongs Utah as the oldest known patelloidean gastropod, of the U­shape, reminiscent of Pollicina crassistesta although Lindberg (2008, 2009) and Vermeij (2017) (Fig. 3C–E), although the dorsal scars in the latter are considered it to be a monoplacophoran mollusc. The smaller and less strongly delimited. holotype within a highly variable sample described by Horný (1965a) introduced the Subclass Cyclomya for Yochelson (1988) displays a well­preserved muscle scar untorted univalves in which the muscle scars formed a (Yochelson 1988, fig. 1.4; Peel & Horný 1999, fig. 14; more or less continuous belt around the apex, including Lindberg 2009, fig. 6) that forms a strongly defined U­ the cyrtonellids and a group of spoon­shaped shells based shape on the lateral and supra­apical surfaces and a more on Archinacella Ulrich & Scofield, 1897. A variety of weakly differentiated band with subsidiary scars across arguments have been proposed by Harper & Rollins the sub­apical surface. The form of the muscle scar is not (1982, 2000), Peel (1990, 1991a, 1991b) and Haszprunar unlike that known in Pollicina crassitesta (Fig. 3C–J, L– (2008) for abandoning the Cyclomya as a formal group N) or some archinacelloids but the height of the shell in but Pollicina may be compared meaningfully in terms of the latter is almost three times its length compared to its muscle scar passing beneath the overhanging apex with about one quarter in Floripatella (Yochelson 1988, archinacelloids (Fig. 4C, D). fig. 4.3). Additionally, the apex lies slightly closer to the The musculature and systematic position of supra­apical margin in Floripatella, in similar fashion to Archinacella as a gastropod were discussed by Golikov many patellids, whereas it overhangs the sub­apical & Starobogatov (1975), Peel (1990, 1991a, 1991b) and margin in Pollicina. Peel & Horný (1999), although it was considered as a Peel & Horný (1999) and Starobogatov & Mazaev monoplacophoran mollusc by others (Knight 1952; (1999) transferred Floripatella to the Family Damilinidae Knight & Yochelson 1960; Runnegar & Jell 1976; Horný, 1961. Damilinids were considered to be typical Wahlman 1992) and to be a mollusc of uncertain position patellids by Horný (1963, 2002) and were classified as by Bouchet et al. (2017). As in Pollicina, the muscle band lottioiden patellogastropods by Bouchet et al. (2017). In passes beneath the apex and across the lateral area, but it Damilina Horný, 1961, as described by Horný (1963), the often fades as it crosses the supra­apical surface. apex is displaced anteriorly from its posterior marginal Archinacelloids differ in having low, spoon­shaped, shells position in Archinacella and Lepetopsis. Unusually, the rather than the tall, cyrtoconic shell of Pollicina. In this U­shaped muscle band in Damilina is broken into respect they are similar in shape to the Carboniferous numerous segments by radial blood vessels, a feature of gastropod Lepetopsis Whitfield, 1882 but also to some extant patellids noted already by Lindström (1884, tergomyans such as Tryblidium and Pilina (Lindström pl. 1, fig. 32) in his description of the tergomyan 1880, 1884; Knight 1941). The cyrtoconic shell form of Tryblidium from the Silurian of Gotland, Sweden. This

31 Estonian Journal of Earth Sciences, 2020, 69, 1, 20–36 repeated bundling of muscle fibres has not been seen in favoured by Evans & Cope (2003). However, these Pollicina crassitexta, although some separation within the morphologies are exotic in the context of Baltica, which scar may occur on the supra­apical surface (Fig. 3C–E). together with the nature of the muscle bands favours a Variation in the degree of bundling in extant patellids was relationship to Archinacella. Ironically, as noted above, a major argument proposed by Harper & Rollins (2000) the placement of Archinacella within Mollusca is also for discriminating between tergomyan and cyclomyan controversial, but opinions expressed by Peel (1990, muscle patterns in Palaeozoic univalves, but many scars 1991a, 1991b) and Peel & Horný (1999) that it is a in Palaeozoic material are sharply defined and evidence superfamily of gastropods (but not patellogastropods) are of bundling is wanting. maintained. The Family Pollinicidae Perner in Koken & Perner, 1925 is therefore placed within the Superfamily Archinacelloidea, established on the basis of the Family CONCLUSIONS Archinacellidae Knight, 1952.

The combination of a bilaterally symmetrical shell with Acknowledgements. This paper results from a visit to Estonia in circum­apertural musculature, comarginal rugae and evi­ 1995 during an exchange programme between Uppsala University dence of frequent repositioning of the apertural margin and the University of Tartu. Mare Isakar (Tartu) kindly arranged indicates that at least some species of Pollicina lived as facilities in Tartu and the loan of specimens from Estonia. Nina limpets clamping against a hard substrate. The slender shell K. Kadlets (St Petersburg) provided access to the collections of the F. N. Chernyshev Central Geological Survey Research and relatively small size of the foot suggest a low energy Museum, St Petersburg, Russia. The gift of photographs and habitat and available specimens with preserved growth replicas of the type material of Pollicina corniculum by the late lines lack substantial evidence of physical or biological Ellis L. Yochelson is acknowledged. David M. Rohr (Alpine, breakage of the apertural margin. Suggestions that Pollicina Texas) and especially Jan Ove R. Ebbestad (Uppsala) are thanked lived as an open coiled gastropod (Yochelson & Webers for comments on the manuscript. The publication costs of this 2006), lying on its lateral area, are rejected. article were covered by the Estonian Academy of Sciences. Placement of Pollicina within molluscan classification is equivocal. Acquisition of the unusually tall and slowly expanding shell, and the interpreted limpet mode of life REFERENCES would have modified the muscle attachment field from the dispersed patterns widely (but no doubt simplistically) Alexander, R. R. & Dietl, G. P. 2003. The fossil record of shell­ considered to be characteristic of tergomyans (Peel breaking predation on marine bivalves and gastropods. In 1991b). On the other hand, a muscle scar pattern of Predator-Prey Interactions in the Fossil Record (Kelley, P. H., archinacelloid type would remain largely unchanged Kowalewski, M. & Hansen, T. A., eds), pp. 141‒176. Kluwer despite the changes in shell morphology. The substantial Academic/Plenum Publishers, New York. change in shell shape from the elongate, spoon­shape of Bandel, K. 1982. Morphologie und Bildung der frühontogenetischen Archinacella to the tall, narrow cone of Pollicina is Gehäuse bei conchiferan Mollusken. Facies, 7, 1–198. Bassler, R. 1915. Bibliographic index of American Ordovician and commensurate with a transition from a mobile habit in Silurian fossils. United States National Museum, Bulletin, Archinacella to the limpet life style of Pollicina. Parallels 92, 1–1521. can be drawn to the development of uncoiled sedentary Billings, E. 1861–1865. Palaeozoic Fossils. Volume 1 Containing Orthonychia within the usually tightly coiled and often Descriptions and Figures of New or Little Known Species mobile platyceratoideans. of Organic Remains from the Silurian Rocks. Geological While the apex in Pollicina is blunt, there is a lack of Survey of Canada, Montreal, 426 pp. conclusive evidence concerning the protoconch and Boucot, A. J. 1990. Evolutionary Paleobiology of Behaviour and earliest growth stages, but this is bilaterally symmetrical Coevolution. Elsevier Science Publishers B.V., Amsterdam, even in modern patellogastropod limpets considered to 725 pp. have been derived from anisotrophic ancestors. The Bouchet, P., Rocroi, J.­P., Frýda, J., Hausdorf, B., Ponder, W., protoconch issue is further complicated in Pollicina by Valdés, Á. & Warén, A. 2005. Classification and nomenclator septation (Evans & Cope 2003) and internal thickening of gastropod families. Malacologia, 47, 1–397. Bouchet, P., Rocroi, J.­P., Hausdorf, B., Kaim, A., Kano, Y., of the shell at the apex. Nützel, A., Parkhaev, P., Schrödl, M. & Strong, E. E. 2017. The frequent development of tall, slender, shells in the Revised classification, nomenclator and typification of inner cratonic late Cambrian–Ordovician carbonate gastropod and monoplacophoran families. Malacologia, 61, successions of Laurentia supports assignment of Pollicina to 1–526. the Class Tergomya, with the curvature of the shell indicating Bowsher, A. L. 1955. Origin and adaptation of platyceratid gastro­ similarity to cyrtonellids rather than hypseloconids in the pods. University of Kansas, Paleontological Contributions, usage of Peel (1991a, 1991b). This is the interpretation Mollusca, 5, 1–11.

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Baltika Ordoviitsiumi molluski Pollicina lihaste kinnitusjäljed, eluviis ja süstemaatika

John S. Peel

Pollicina on iseloomuliku ühepoolmelise kojaga, kuid harva esinev molluskite perekond, mida esmakordselt kirjeldati Balti regiooni Kesk­Ordoviitsiumist (Darriwili lade). Tema sihvakas, bilateraal­sümmeetriline koda laieneb aeglaselt ja on kõverdatud kuni 90­kraadise nurga all, kuid sirgeneb viimastes kasvufaasides. Artiklis on uuesti kirjeldatud Peterburi regioonist pärit perekonna tüüpliiki Pollicina corniculum ja Põhja­Eestis Tallinna piirkonnas levinud liiki Pollicina crassitesta. P. crassitesta lihaste kinnitusjäljed moodustavad valatisena säilinud kivististel pideva ümmarguse rea koja tipu ja ava vahel. Koja sümmeetria, ortokliiniline ava, lihaste kinnitusjäljed ja ava serva sagedane nihkumine, mida tõendavad kasvujoonte dislokatsioonid, näitavad, et Pollicina elas substraadi vastu klammerdunult nagu mereliud. Artiklis on ümber lükatud varasem arvamus, et tegemist oli settel lebava vähe keerdunud gastropoodiga. Nagu enamiku Ordoviitsiumi mereliualaadsete fossiilide puhul, ei ole ka Pollicina süstemaatiline kuuluvus klassi Tergomya või klassi Gastropoda ühemõtteline. Vaatamata sarnasustele perekonnaga Cyrtolites, paigutatakse antud töös Pollicina siiski gastropoodide hulka koos sugukonna Archinacellidae esindajatega.

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